In the Traditio Legis iconographic type, an enthroned Christ hands symbols of authority to Saints Peter and Paul, who stand at his side.
The earliest examples are found on paleo-Christian sarcophagi. In these, the actual symbols chosen will vary. In this example, St. Paul receives a scroll and St. Peter holds some sort of bowl. But in this one it is St. Peter who receives the scroll. On his shoulder he carries some sort of rod, which may be a damaged cross. We see the same attempt to portray a cross (or conceivably a sceptre or fasces) in this fragment in the Vatican Museums. In the sarcophagus shown above, from a century or so later, the object is certainly a cross, and the symbol of authority he has been given is a key. The key alludes to Matthew 16:19, "I will give to thee the keys of the kingdom of heaven."
In that same sarcophagus the symbol of authority given to Paul is a scroll. This pattern of giving keys to Peter and a scroll to Paul comes to dominate the iconography until the 12th century, as in the first example on the right. In the second example on the right, a 12th-century restoration of a 9th-century original, Peter receives keys and Paul a book.
After the 12th century, this iconographic type loses favor. In the West, it will be succeeded by narrative paintings picturing the text from Matthew without reference to St. Paul (example).
Prepared in 2016 by Richard Stracke, Emeritus Professor of English, Augusta University
A sarcophagus in Sant'Apollinare in Classe, near Ravenna. Follow this link for the description page.